Problem with fireflies and denoise on surfaces
These "splotches" are because of Blender's default denoiser. They can be removed by increasing the denoising radius (something like 14 px or so). However, it has other serious side effects (eg, incorrectly blurring the sharp shadows of things like individual leaves of a plant). Try using Intel's Open Image Denoiser in compositor, or Nvidia Optix denoiser (https://www.youtube.com/watch?v=eGAjsSNtX6E) instead. They're relatively better denoisers.